The present invention generally relates to a method for microelectronics fabrication and the microelectronic structure formed and more particularly, relates to a method for forming amorphous silicon films on a single crystal silicon substrate and the semiconductor structure formed.
In semiconductor fabrication technology, thin films of silicon that does not have a single crystal structure are frequently used in forming microelectronic devices. These non-single crystal silicon includes amorphous silicon and polycrystalline silicon. For instance, amorphous silicon is widely used in the formation of TFT (thin film transistor) devices and for CMOS gates when a dual gate process is used. Polysilicon films have numerous applications in forming microelectronic devices including gate conductors in MOSFETs when the films are heavily doped, as a dopant source for contacts of shallow junctions in bipolar transistors, and as a conducting layer for interconnections. The amorphous and polysilicon thin films can be formed by a variety of chemical vapor deposition techniques, i.e. LPCVD (low pressure chemical vapor deposition), PECVD (plasma enhanced chemical vapor deposition) and RTCVD (room temperature chemical vapor deposition). The amorphous silicon films can also be formed by a physical vapor deposition technique. The chemical vapor deposition technique is carried out typically by the decomposition process of a silicon-containing gas, such as silane (SiH4) or disilane (Si2H6) with or without a diluting gas of argon. For a LPCVD process, the typical deposition temperature runs between 500xc2x0 C. and 700xc2x0 C. at a typical chamber pressure from 100 mTorr and 500 Torr. The structure of the silicon film deposited, i.e. of either amorphous or polycrystalline, depends mainly on the deposition temperature. For instance, amorphous silicon films are normally deposited at a deposition temperature below 575xc2x0 C. A transition between the amorphous structure and the polycrystalline structure occurs between the temperature of 575xc2x0 C. and 600xc2x0 C. such that polycrystalline silicon is normally deposited at a temperature higher than 580xc2x0 C. The amorphous silicon structure which has no detectable crystallinity can be converted to polycrystalline structures upon further heat treatment above 600xc2x0 C. The grain size increases upon heat treatment at even higher temperatures.
When an amorphous silicon film is directly deposited on top of a single crystal silicon substrate, problems of surface imperfection is frequently observed. The surface imperfection appears in the form of unevenness due to irregularly formed protrusions, or islands on the top surface of the amorphous silicon film layer. The obtaining of a smoothly deposited surface of the amorphous silicon film on top of a single crystal silicon substrate is therefore impossible. This leads to difficulties in using amorphous silicon films in various microelectronic applications incorporating the MEMS (micro-electro-mechanical-system) technique such as the fabrication of a cantilever beam for a micro-relay, or in the fabrication of a suspended mirror in a digital mirror display, or in a glass-to-glass bonding application, or in the solar cell application. In the above discussed microelectronic applications that incorporate the MEMS technique, an amorphous silicon film that has a smooth top surface and a minimum defect density on the top surface must be obtained.
For instance, in the application of the cantilever beam for the micro-relay and the suspended mirror in the digital mirror display, a key element for the MEMS process is the capability of growing a relatively thick amorphous silicon film that has a smooth top surface. The amorphous silicon film is frequently used as a sacrificial layer, i.e. to be later removed, in these applications. While currently the amorphous silicon film may be grown by LPCVD, PECVD or PVD, various processing difficulties exist that prevent the successful implementation of these deposition techniques. For instance, in order to form an amorphous silicon film that has low surface defect density by the LPCVD method, the deposition temperature must be around 550xc2x0 C. which is not compatible with the CMOS process. On the other hand, when the amorphous silicon film is deposited by the PVD technique, the film produced has poor adhesion and thus greatly limits its deposition thickness.
It is therefore an object of the present invention to provide a method for forming amorphous silicon films on single crystal silicon substrates that does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method for forming amorphous silicon films that have low defect density and a smooth top surface on a single crystal silicon substrate.
It is a further object of the present invention to provide a method for forming amorphous silicon films-on single crystal silicon substrates that are compatible with a front-end CMOS process.
It is another further object of the present invention to provide a method for forming amorphous silicon films on single crystal silicon substrates that does not require an annealing process after the deposition.
It is still another object of the present invention to provide a method for forming amorphous silicon films on single crystal silicon substrates that can be integrated with a MEMS process.
It is yet another object of the present invention to provide a semiconductor structure of an amorphous silicon film deposited on a single crystal silicon substrate with a smooth top surface on the amorphous silicon film.
It is still another further object of the present invention to provide a semiconductor structure on a single crystal silicon substrate that includes a single crystal silicon substrate, a buffer layer on top of the substrate, and an amorphous silicon film deposited on top of the buffer layer.
It is yet another further object of the present invention to provide a semiconductor structure that is built on a single crystal silicon substrate including a buffer layer of silicon oxide, silicon nitride, silicon carbide or metal in-between an amorphous silicon film and a single crystal silicon substrate.
In accordance with the present invention, a method for forming amorphous silicon films that have low defect density on a single crystal silicon substrate and structures formed are provided.
In a preferred embodiment, a method for forming amorphous silicon films with low defect density on single crystal silicon can be carried out by the operating steps of providing a single crystal silicon substrate; depositing a buffer layer in a material capable of surviving a process temperature of 500xc2x0 C. on top of the single crystal silicon substrate, the buffer layer has a thickness of at least 500 xc3x85; and depositing an amorphous silicon film on a top surface of the buffer layer.
The method for forming amorphous silicon films with low defect density on single crystal silicon may further include the step of selecting the material for the buffer layer from the group consisting of silicon oxide, silicon nitride, silicon carbide and metals, or the step of depositing the buffer layer to a thickness between about 500 xc3x85 and about 1500 xc3x85, or the step of depositing the buffer layer by a technique selected from the group consisting of chemical vapor deposition, plasma enhanced chemical vapor deposition and physical vapor deposition. The method may further include the step of depositing the amorphous silicon film by a technique selected from the group consisting of plasma enhanced chemical vapor deposition, low pressure chemical vapor deposition, room temperature chemical vapor deposition and physical vapor deposition, or the step of depositing the amorphous silicon film to a thickness between about 1 xcexcm and about 10 xcexcm, or the step of depositing the amorphous silicon film in a deposition chamber fabricated of stainless steel.
The method for forming amorphous silicon films with low defect density on single crystal silicon may further include the step of depositing the amorphous silicon film in a plasma enhanced chemical vapor deposition chamber at a deposition temperature between about 275xc2x0 C. and about 375xc2x0 C., or the step of depositing the amorphous silicon film in a plasma enhanced chemical vapor deposition chamber at a deposition power between about 50W and about 400W, or the step of depositing the amorphous silicon film at a chamber pressure between about 400 mTorr and about 900 mTorr, or the step of depositing the amorphous silicon film in a chemical vapor deposition chamber by flowing in a precursor selected from the group consisting of SiH4 and SiH4/argon mixture. The method may further include the step of annealing by laser energy the amorphous silicon film into a polysilicon film.
The invention is further directed to a semiconductor structure that is built on a single crystal silicon substrate which includes a single crystal silicon substrate; a buffer layer on top of the single crystal silicon substrate formed of a material capable of surviving a process temperature of 500xc2x0 C., the buffer layer has a thickness of at least 500 xc3x85; and an amorphous silicon film that has substantially no-surface defect on top of the buffer layer.
The semiconductor structure that is built on a single crystal silicon substrate may further include a polysilicon film on top of the buffer layer in place of the amorphous silicon film. The buffer layer may be formed of a material selected from the group consisting of silicon oxide, silicon nitride, silicon carbide and metals. The buffer layer may have a thickness between about 500 xc3x85 and about 1500 xc3x85. The amorphous silicon film may have a thickness between about 1 xcexcm and about 10 xcexcm. The amorphous silicon film may be formed in a cantilever beam for a micro-relay device, or may be formed on a suspended mirror for a digital mirror display device, or may be formed and converted to polysilicon as an electrode in a solar cell.